Metal fibre concrete, cementitious matrix and pre-mixes for preparing matrix and concrete

ABSTRACT

The invention concerns a concrete obtained by mixing with water, cement, granular elements, elements with puzzolanic reaction, constituents capable of improving the toughness of the matrix, metal fibers and at least a dispersing agent, in specified conditions and proportions. Said concrete has improved properties compared to prior art concrete comprising metal fibers.

The present invention relates to the field of concretes, moreparticularly to fibre-reinforced concretes. The main subject of theinvention is an improved concrete, especially making it possible tomanufacture elements of civil engineering structures, intended forconstructing buildings and highway structures, and having propertiessuperior to those of elements in the prior art. In particular, thepresent invention aims to obtain, for structural concretes, mechanicalbehaviour which is tough and ductile at the same time.

Structural analysis of concretes has shown that their mechanicalproperties are intimately linked to the presence of structural defects.Several types of defects distinguishable by their size may be observedin these concretes when they are subjected to mechanical loads.

On a smaller scale, the defect called microporosity is observed inconcrete. This consists of pores, called capillaries, emanating from theintergranular spaces initially present in the fresh paste. Their sizevaries between 50 nm and a few μm.

On the next scale up, microcracking defects are observed. These aremicrocracks having openings ranging from 1 μm to a few hundreds of Am.They are non-coalescent, that is to say that they do not form acontinuous path through the structure. They are mainly due to theheterogeneous character of concrete, the aggregate having mechanical andphysical properties different from those of the binder/cement. Thesemicrocracks appear during mechanical loading. This type of defect is amajor reason for the poor mechanical properties of concrete in tensionand for its brittle character.

On the final scale, macrocracking defects are observed. The opening ofthese cracks varies from a few hundreds of μm to a few mm. These cracksare coalescent.

Major defects several millimetres in size may also be observed, thesebeing due to poor preparation of the concrete (entrained air, faults infilling).

Solutions have been suggested either for decreasing the presence ofthese various defects or for reducing their effects on the mechanicalproperties of the concrete.

In order to improve the mechanical properties of concretes, it has beenproposed to replace the sand of the cementitious matrix by other,higher-performance, constituents, but the cost of the concrete rises toan unacceptable level for it to be conceivably used widely in civilengineering because of the economic constraints which burden this field.

It has also been proposed to incorporate high-hardness aggregate intothe concrete composition, but the amounts involved in order to achievethe desired performance also increase the manufacturing cost of theconcrete excessively, given the high cost of such aggregate.

It has also been proposed to improve, sometimes spectacularly, certainmechanical properties of concrete by incorporating into it a highcontent of reinforcing fibres, namely, typically, a content of 10 to 15%by volume, but this content not only has a very significant effect onthe manufacturing cost of the concrete but also makes its mixing,homogenization and possibly its casting too difficult or too critical tobe applicable in civil engineering, especially under the workingconditions of a construction site.

Also, it has been possible to partially control the microporosity bydecreasing the water-to-cement weight ratio and by using plasticizers.The use of fine fillers, especially pozzolanic-reaction fillers, hasalso allowed the size of the micropores to be reduced.

However, the organization of the aggregate skeleton by the usual methodsdoes not make it possible to obtain concrete with a satisfactoryrheology under acceptable civil engineering operating conditions (poorlydispersed fibres, microstructural defects, etc.).

Microcracking itself has been greatly reduced by:

improving the homogeneity of the concrete, for example by limiting thesize of the aggregate to 800 μm;

improving the compactness of the material (aggregate optimization andoptional pressing before and during the setting phase);

carrying out heat treatments after setting.

With regard to macrocracking, this may be controlled by the use of metalfibres, but with the operating difficulties mentioned above.

By way of an illustrative document of the prior art, mention may be madeof Patent Application WO-A-95/01316 which relates to ametal-fibre-reinforced concrete in which the fibre content is controlledand the fibre dimensions are chosen in defined proportions with respectto those of the aggregate particles.

This fibre-reinforced concrete comprises cement, aggregate particles,fine pozzolanic-reaction particles and metal fibres. The aggregateparticles must have a maximum size D of at most 800 μm, the fibres musthave an individual length 1 of between 4 and 20 mm and the ratio R ofthe average length L of the fibres to D must be at least equal to 10,the fibre content being such that the fibres occupy a volume of from 1to 4% of the volume of the concrete after it has set.

The concrete obtained exhibits ductile behaviour or undergoespseudo-work-hardening.

There is still a need to remove the aforementioned defects or to greatlyreduce their effects, especially microcracks, as it may be seen that thework described in the prior art serves mainly to prevent the developmentof macrocracks and not of microcracks; microcracks are then onlypartially stabilized and develop under load.

The object of the present invention is to provide a concrete containingmetal reinforcing fibres and having improved properties compared withsimilar concretes of the prior art.

Improved properties should be understood to mean both mechanicalproperties that are superior to those of known fibre-reinforcedconcretes and properties that are at least equal to those of knownfibre-reinforced concretes, but these properties being achievable on anindustrial scale in a constant and reproducible manner.

Another object of the present invention is to increase the stress levelat which the first damage (i.e. microcracks) appears in the concrete andthus to increase the range of use of the concrete, namely the linearelastic behaviour of the concrete.

Yet another object of the present invention is to improve the workhardening of the concrete beyond the first damage by controlling thepropagation of macrocracks. The purpose of the invention is thus toincrease the range of use of the concrete beyond the first damage byimproving the ductile behaviour of the concrete.

Another object of the invention is also to improve, by a synergy effectbetween the cementitious matrix and the fibres, the behaviour of theconcrete both with respect to the appearance of microcracks and to thepropagation of macrocracks.

“Cementitious matrix” should be understood to mean the hardenedcementitious composition apart from the metal fibres.

Yet another object of the present invention, which is particularlyimportant for obtaining concrete structures which, because of their sizeor the work site conditions, could not undergo a heat treatment, is toobtain, under improved conditions over the prior art and especially attemperatures close to ambient temperature (20° C.), a concrete havingmechanical properties (in the sense mentioned above) at least equal tothose which can only be obtained at the cost of a heat treatment in thecase of the best known fibre-reinforced concretes.

In addition, the subject of the present invention is the cementitiousmatrix, which allows the concrete of the invention to be produced, andthe premixes which comprise all or some of the constituents necessaryfor preparing this matrix or the concrete.

In its general form, the invention relates to a concrete consisting of ahardened cementitious matrix in which metal fibres are dispersed,obtained by mixing, with water, a composition which comprises, apartfrom the fibres:

(a) cement;

(b) aggregate particles having a maximum particle size Dmax of at most 2mm, preferably at most 1 mm;

(c) pozzolanic-reaction particles having an elementary particle size ofat most 1 μm, preferably at most 0.5 μm;

(d) constituents capable of improving the toughness of the matrix, thesebeing chosen from acicular or flaky particles having an average size ofat most 1 mm and present in a proportion by volume of between 2.5 and35% of the combined volume of the aggregate particles (b) and of thepozzolanic-reaction particles (c);

(e) at least one dispersing agent; and satisfying the followingconditions:

(1) the percentage by weight of water w with respect to the combinedweight of the cement (a) and of the particles (c) is in the range 8-24%;

(2) the fibres have an individual length 1 of at least 2 mm and an l/dratio of at least 20, d being the diameter of the fibres;

(3) the ratio R of the average length L of the fibres to the maximumparticle size Dmax of the aggregate particles is at least 10;

(4) the amount of fibres is such that their volume is less than 4% andpreferably less than 3.5% of the volume of the concrete after it hasset.

Thus, by virtue of a novel design of the aggregate skeleton and of itsrelationship with the reinforcing fibres, this approach solves theproblem posed with this rheology/mechanical properties compromise.

The properties of the concrete according to the invention are notappreciably changed if aggregate particles (b) having a particle sizeexceeding 2 mm are also used within the matrix but in a proportion whichdoes not exceed 25% of the volume of the combination of constituents(a)+(b)+(c)+(d).

The presence of this aggregate class in such a proportion may beregarded as a filler which does not contribute to the mechanicalperformance of the material in so far as:

the D50 particle size of the combination of constituents (a), (b), (c)and (d) is at most 200 μm, preferably at most 150 μm; and

the ratio R of the average length L of the fibres to the D75 particlesize of the combination of constituents (a), (b), (c) and (d) is atleast 5, preferably at least 10.

D75 particle size and D50 particle size should be understood to mean,respectively, the sizes of the screens whose undersize constitutes 75%and 50%, respectively, of the total volume of the particles.

The invention therefore also relates to a concrete consisting of ahardened cementitious matrix in which metal fibres are dispersed,obtained by mixing, with water, a composition which comprises, apartfrom the fibres:

(a) cement;

(b) aggregate particles;

(c) pozzolanic-reaction particles having an elementary particle size ofat most 1 μm, preferably at most 0.5 μm;

(d) constituents capable of improving the toughness of the matrix, thesebeing chosen from acicular or flaky particles having an average size ofat most 1 mm and present in a proportion by volume of between 2.5 and35% of the combined volume of the aggregate particles (b) and of thepozzolanic-reaction particles (c);

(e) at least one dispersing agent; and satisfying the followingconditions:

(1) the percentage by weight of water W with respect to the combinedweight of the cement (a) and of the particles (c) is in the range 8-24%;

(2) the fibres have an individual length 1 of at least 2 mm and an l/dratio of at least 20, d being the diameter of the fibres;

(3) the ratio R of the average length L of the fibres to the D75particle size of the combination of constituents (a), (b), (c) and (d)is at least 5, preferably at least 10;

(4) the amount of fibres is such that their volume is less than 4% andpreferably less than 3.5% of the volume of the concrete after it hasset;

(5) the combination of the constituents (a), (b), (c) and (d) has a D75particle size of at most 2 mm, preferably at most 1 mm, and a D50particle size of at most 150 μm, preferably at most 100 μm.

Conditions (3) and (5) apply to all the solid constituents (a), (b), (c)and (d) all together, excluding fibres, and not for each constituenttaken individually.

Preferably, the toughness of the cementitious matrix is at least 15J/m², advantageously at least 20 J/m².

The toughness is expressed either in terms of stress (stress intensityfactor: K_(c)) or in terms of energy (critical strain energy releaserate: G_(c)), using the formalism of linear fracture mechanics.

The measurement methods used to determine the toughness of thecementitious matrix will be described below in the part of thedescription pertaining to the examples.

The toughness of the cementitious matrix is obtained by adding to thecementitious composition particles (d) of average size of at most 1 mm,preferably at most 500 μm, these being in acicular form or in the formof flakes. They are present in a proportion by volume lying in the range2.5-35%, in particular in the range 5-25%, of the combined volume of theaggregate particles (b) and of the pozzolanic-reaction particles (c).

On account of their function to improve the toughness of the matrix, thesaid particles will be called hereafter in the description “reinforcingparticles”.

The term “size” of the reinforcing particles should be understood tomean the size of their largest dimension (especially the length in thecase of those of acicular form).

These may be natural or synthetic products.

The reinforcing particles of acicular form may be chosen from amongwollastonite fibres, bauxite fibres, mullite fibres, potassium titanatefibres, silicon carbide fibres, cellulose or cellulose-derivativefibres, such as cellulose acetate, carbon fibres, calcium carbonatefibres, hydroxyapatite fibres and other calcium phosphates, or derivedproducts obtained by grinding the said fibres and mixtures of the saidfibres.

Preferably, reinforcing particles are used whose acicularity, expressedby the length/diameter ratio, is at least 3 and preferably at least 5.

Wollastonite fibres have given good results. Thus, the presence ofwollastonite fibres in the cementitious matrix leads to a reduction inthe microporosity. This surprising effect is particularly apparent inthe case of concretes which have undergone 20° C. maturing (see below).

The reinforcing particles in the form of flakes may be chosen from amongmica flakes, talc flakes, mixed silicate (clay) flakes, vermiculiteflakes, alumina flakes and mixed aluminate or silicate flakes andmixtures of the said flakes.

Mica flakes have given good results.

It is possible to use combinations of these various forms or types ofreinforcing particles in the composition of the concrete according tothe invention.

At least some of these reinforcing particles may have, on their surface,a polymeric organic coating which comprises a latex or is obtained fromat least one of the following compounds: polyvinyl alcohol, silanes,siliconates, siloxane resins, polyorganosiloxanes or products fromreaction between (1) at least one carboxylic acid containing from 3 to22 carbon atoms, (2) at least one polyfunctional aliphatic or aromaticamine or substituted amine, containing from 2 to 25 carbon atoms and (3)a crosslinking agent which is a water-soluble metal complex containingat least one metal chosen from among: zinc, aluminium, titanium, copper,chromium, iron, zirconium and lead; this product is more particularlydescribed in Application EP-A-0,372,804.

The thickness of this coating may vary between 0.01 and 10 μm,preferably between 0.1 and 1 μm.

The latices may be chosen from among styrene-butadiene latices, acryliclatices, styrene-acrylic latices, methacrylic latices and carbonylatedand phosphonated latices. Latices having functional groups which complexwith calcium are preferred.

The polymeric organic coating may be obtained by treating, in afluidized bed or by using a FORBERG-type mixer, the reinforcingparticles in the presence of one of the compounds defined above.

The following compounds are preferred: H240 polyorganosiloxane, Manalox403/60/WS and WB LS 14 and Rhodorsil 878, 865 and 1830 PX siloxaneresins, all sold by Rhodia-Chimie, and potassium siliconates.

This type of treatment is particularly recommended for reinforcingparticles which are naturally occurring substances.

With regard to the metal fibres, these may be metal fibres chosen fromamong steel fibres, such as high-strength steel fibres, amorphous steelfibres or stainless steel fibres. Optionally, the steel fibres may becoated with a non-ferrous metal such as copper, zinc, nickel (or theiralloys).

The individual length 1 of the metal fibres is at least 2 mm and ispreferably in the 10-30 mm range. The l/d ratio is at least 20, andpreferably at most 200, d being the diameter of the fibres.

Fibres having a variable geometry may be used: they may be crimped,corrugated or hooked at the ends. The roughness of the fibres may alsobe varied and/or fibres of variable cross-section may be used; thefibres may be obtained by any suitable technique, including by braidingor cabling several metal wires, forming a twisted assembly.

The fibre content is such that the fibres occupy a volume of less than4%, and preferably of less than 3.5%, of the volume of the concreteafter it has set.

Advantageously, the average bonding stress of the fibres in the hardenedcementitious matrix must be at least 10 MPa, preferably at least 15 MPa.This stress is determined by a test comprising the extraction of asingle fibre embedded in a block of concrete, as will be describedbelow.

It has been observed that the concretes according to the invention,having both such a fibre-bonding stress and a high matrix toughness,(preferably of at least 15 J/m²), result in superior mechanicalperformance, by synergy between these two properties.

The level of fibre/matrix bonding may be controlled by several means,which may be used individually or simultaneously.

According to a first means, the bonding of the fibres in thecementitious matrix may be achieved by treating the surface of thefibres. This fibre treatment may be carried out by at least one of thefollowing processes:

fibre etching;

deposition of a mineral compound on the fibres, especially by depositingsilica or a metal phosphate.

The etching may be carried out, for example, by bringing the fibres intocontact with an acid, followed by neutralization.

Silica may be deposited by bringing the fibres into contact with siliconcompounds, such as silanes, siliconates or silica sols.

In general, a metal phosphate is deposited using a phosphatizingprocess, which consists in introducing prepickled metal fibres into anaqueous solution comprising a metal phosphate, preferably manganesephosphate or zinc phosphate, and then in filtering the solution in orderto recover the fibres. Next, the fibres are rinsed, neutralized and thenrinsed again. Unlike the usual phosphatizing process, the fibresobtained do not have to undergo grease-type finishing; however, they maybe optionally impregnated with an additive either in order to provideanticorrosion protection or to make it easier for them to be processedwith the cementitious medium. The phosphatizing treatment may also becarried out by coating or spraying the metal phosphate solution onto thefibres.

Any type of phosphatizing process may be used—reference may be made onthis subject to the treatments described in the article by G. LORINentitled “The Phosphatizing of Metals” (1973), Pub. Eyrolles.

According to a second means, the bonding stress of the fibres in thecementitious matrix may be achieved by introducing into the compositionat least one of the following compounds: silica compounds comprisingmostly silica, precipitated calcium carbonate, polyvinyl alcohol inaqueous solution, a latex or a mixture of the said compounds.

The phrase “silica compound comprising mostly silica” should beunderstood here to mean synthetic products chosen from amongprecipitated silicas, silica sols, pyrogenic silicas (of the Aerosiltype), aluminosilicates, for example Tixosil 28 sold by Rhodia Chimie,or clay-type products (either natural or derived), for examplesmectites, magnesium silicates, sepiolites and montmorillonites.

It is preferred to use at least one precipitated silica.

Precipitated silica should be understood here to mean a silica obtainedby precipitation from the reaction of an alkali metal silicate with anacid, generally an inorganic acid, with a suitable pH of theprecipitation medium, in particular a basic, neutral or slightly acidpH; any method may be used to prepare the silica (addition of acid to asilicate sediment, total or partial simultaneous addition of acid or ofsilicate to a water or silicate-solution sediment, etc.), the methodbeing chosen depending on the type of silica that it is desired toobtain; after the precipitation step there generally follows a step ofseparating the silica from the reaction mixture using any known means,for example a filter press or a vacuum filter; a filter cake is thuscollected, which is washed if necessary; this cake may, optionally aftercrumbling, be dried by any known means, especially by spray drying, andthen optionally ground and/or agglomerated.

In general, the amount of precipitated silica introduced is between 0.1%and 5% by weight, expressed as dry matter, with respect to the totalweight of the concrete. Above 5%, rheologie problems during preparationof the mortar usually arise.

Preferably, the precipitated silica is introduced into the compositionin the form of an aqueous suspension. This may especially be an aqueoussilica suspension having:

a solids content of between 10 and 40% by weight;

a viscosity of less than 4×10⁻² Pa.s for a shear of 50 s⁻¹;

an amount of silica contained in the supernatant liquid of the saidsuspension after centrifuging at 7500 rpm for 30 minutes of more than50% of the weight of the silica contained in the suspension.

This suspension is more particularly described in Patent ApplicationWO-A-96/01787. The Rhoximat CS 60 SL silica suspension sold by RhodiaChimie is particularly suitable for this type of concrete.

The cement (a) of the composition according to the invention isadvantageously a Portland cement, such as the Portland cements CPA PMES,HP, HPR, CEM I PMES, 52.5 or 52.5R or HTS (high silica content).

The aggregate particles (b) are essentially screened or ground sands ormixtures of sands, which advantageously may comprise silicious sands,particularly quartz flour.

The maximum particle size D100 or Dmax of these particles is preferablyat most 6 mm.

These aggregate particles are generally present in an amount of 20 to60% by weight of the cementitious matrix, preferably from 25 to 50% byweight of the said matrix.

The fine pozzolanic-reaction particles (c) have an elementary particlesize of at least 0.1 μm and at most 1 μm, preferably at most 0.5 μm.They may be chosen from among silica compounds, especially silica fume,fly ash, blast-furnace slag and clay derivatives, such as kaolin. Thesilica may be a silica fume coming from the zirconium industry ratherthan a silica fume coming from the silicon industry.

The water-cement weight ratio, conventional in concrete technology, mayvary when cement substitutes, especially pozzolanic-reaction particles,are used. For the needs of the present invention, the weight ratio ofthe amount of water W with respect to the combined weight of the cementto the pozzolanic-reaction particles has therefore been defined. Thisratio, thus defined, is between approximately 8 and 24%, preferablybetween approximately 13 and 20%. However, in the description of theexamples, the water-to-cement ratio W/C will be used.

The composition according to the invention also comprises at least onedispersing agent (e). This dispersing agent is generally a plasticizer.The plasticizer may be chosen from among: lignosulphonates, casein,polynaphthalenes, particularly polynaphthalene-sulphonates of alkalimetals, formaldehyde derivatives, polyacrylates of alkali metals,polycarboxylates of alkali metals and grafted polyethylene oxides. Ingeneral, the composition according to the invention comprises from 0.5to 2.5 parts by weight of plasticizer per 100 parts by weight of cement.

Other additives may be added to the composition according to theinvention, for example an anti-foam agent. By way of example, anti-foamagents based on polydimethylsiloxane or on propylene glycol may be used.

Among agents of this type, mention may be made especially of siliconesin the form of a solution or in the form of a solid or, preferably, inthe form of a resin, an oil or an emulsion, preferably in water. Mostparticularly suitable are silicones essentially comprising M repeatunits (RsiO_(0.5)) and D repeat units (R₂SiO). In these formulae, theradicals R, which may be identical or different, are more particularlychosen from among hydrogen and alkyl radicals comprising 1 to 8 carbonatoms, the methyl radical being preferred. The number of repeat units ispreferably in the 30 to 120 range.

The amount of such an agent in the composition is generally at most 5parts by weight per 100 parts of cement.

All the sizes of the particles are measured by TEM (transmissionelectron microscopy) or SEM (scanning electron microscopy).

The matrix may also contain other ingredients as long as they do notprejudice the expected performance of the concrete.

The concrete may be obtained according to any process known to thoseskilled in the art, especially by mixing the solid constituents withwater, forming (moulding, casting, injection, pumping, extrusion,calendering) and then hardening.

For example, in order to prepare the concrete the constituents of thematrix and the reinforcing fibres are mixed with the suitable amount ofwater.

Advantageously, the following order of mixing is respected:

mixing of the pulverulent constituents of the matrix (for example for 2minutes);

introduction of the water and a fraction, for example half, of theadmixtures;

mixing (for example for 1 minute);

introduction of the remaining fraction of the admixtures;

mixing (for example for 3 minutes);

introduction of the reinforcing fibres and the additional constituents;

mixing (for example for 2 minutes).

The concrete then undergoes maturing between 20° C. and 100° C. for thetime necessary to obtain the desired mechanical properties.

Surprisingly, it has been found that maturing at a temperature close toambient temperature gave good results, this being so by virtue of thechoice of constituents in the composition of the concrete.

In this case, the concrete is left to mature at, for example, atemperature close to 20° C.

The maturing process may also involve a heat treatment between 60 and100° C. at normal pressure on the hardened concrete.

The concrete obtained may be especially subjected to a heat treatmentbetween 60 and 100° C. for between 6 hours and 4 days, with the optimumtime being about 2 days and the treatment starting after the end of themixture setting phase or at least one day after the onset of setting. Ingeneral, treatment times of 6 to 72 hours are sufficient within theaforementioned temperature range.

The heat treatment is carried out in a dry or wet environment or carriedout according to cycles alternating between the two environments, forexample 24 hours in a wet environment followed by 24 hours in a dryenvironment.

This heat treatment is implemented on concretes which have completedtheir setting phase, these preferably being aged for at least one dayand better still aged for at least approximately 7 days.

The addition of quartz powder may be useful when the concrete issubjected to the aforementioned heat treatment.

The concrete may be pretensioned, by bonded wires or by bonded tendons,or post-tensioned, by single unbonded tendons or by cables or by sheathsbars, the cable consisting of an assembly of wires or consisting oftendons.

The prestressing, whether in the form of pretensioning or in the form ofpost-tensioning, is particularly well suited to products made of theconcrete according to the invention.

This is because metal prestressing cables always have a very high,ill-used, tensile strength since the brittleness of the matrix whichcontains them does not allow the dimensions of the concrete structuralelements to be optimized.

Progress has already been made in terms of the use of high-performanceconcretes; in the case of the concrete according to the invention, thematerial is homogeneously reinforced by metal fibres allowing it toachieve high mechanical performance in conjunction with ductility. Theprestressing of this material, by means of cables or tendons, whateverthe pretensioning mode, is then used almost to its full amount, therebycreating prestressed concrete elements that are very strong both intension and in bending, and are therefore optimized.

The reduction in volume obtained, because of this increase in mechanicalstrength, can produce very light prefabricated elements. Consequently,there is then the possibility of having long-span concrete elements thatare easily transportable because of their lightness; this isparticularly well suited to the construction of large structures inwhich the use of post-tensioning is widely employed. In the case of thistype of structure, the solution provides particularly favourable savingsto be made in terms of work-site duration times and assembly.

In addition, in the case of a thermal cure, the use of pretensioning orpost-tensioning significantly reduces shrinkage.

This property is particularly desirable and all of the above advantagesassociated with the very low permeability of the product—highlyadvantageous in the case of durability and maintenance of structuresover time—mean that this material may validly be substituted forstructures made of steel.

The concretes obtained according to the present invention generally havea direct tensile strength R_(t) of at least 12 MPa.

They may also have a flexural strength R_(f) in 4-point bending of atleast 25 MPa, a compressive strength R_(c) of at least 150 MPa and afracture energy W_(f) of at least 2500 J/m².

The invention also relates to the cementitious matrix intended forobtaining and for employing the concrete defined above.

Finally, the invention relates to premixes comprising all or some of theconstituents necessary to prepare the concrete and the matrix definedabove.

BRIEF DESCRIPTION OF THE DRAWINGS

FIG. 1 is a graph obtained from the bending tests with the stress values(in MPa) plotted on the y-axis and the deflection values (in mm) plottedon the x-axis for concrete specimens with a W/C ratio of 0.24 andmaturing at 20° C., respectively with wollastonite (curves 12.1, 12.2and 12.3) and without wollastonite (curves 11.1, 11.2 and 11.3).

FIG. 2 is a graph similar to FIG. 1 but for concrete specimens of thesame composition with a 90° C. heat treatment: with wollastonite (curves10.1, 10.2 and 10.3) and without wollastonite (curves 9.1, 9.2 and 9.3).

FIG. 3 is a graph obtained by tensile tests on concrete specimensrelating to untreated steel fibres, with a W/C ratio of 0.20 and a 90°C. heat treatment, respectively with precipitated silica (curves 20.1,20.2 and 20.3) and without precipitated silica (curves 20.4 and 20.5).

FIG. 4 is a graph obtained from bending tests for three specimens with aW/C ratio of 0.25 and a 90° C. heat treatment respectively withsurface-treated fibres (curves 16.1 and 16.2) and untreated fibres(curve 15.1). Plotted on the y-axis are the values of the bending stress(in MPa) and plotted on the x-axis are the values of the deflection (inmm).

FIGS. 5 to 7 show the porosity of concrete specimens, this beingdetermined by the mercury intrusion technique: plotted on the y-axis isthe cumulative volume (ml/g) and on the x-axis the diameter (inmicrometres) of the pores.

FIG. 5 corresponds to a concrete specimen (Example 1) which hasundergone 20° C. maturing.

FIG. 6 corresponds to a concrete specimen (Example 2) which hasundergone a 90° C. heat treatment.

FIG. 7 corresponds to a concrete specimen containing wollastonite(Example 3) which has undergone 20° C. maturing.

FIG. 8 is a graph resulting from ²⁹Si nuclear magnetic resonanceanalysis of a concrete according to the invention containingwollastonite and matured at 20° C. (curve 23) compared with two sameconcretes having the same composition but not containing wollastonite,one having been subjected to a 90° C. heat treatment (curve 22) and theother having been matured at 20° C. (curve 24). It may be seen thatthere is little difference between the two curves 22 and 23, as regardspeaks Q2. These peaks, relating to the double bonds of SiO⁴ radicals,are all the more intense the longer the hydrate chains. It may thereforebe concluded that adding wollastonite makes it possible to obtain at 20°C. a hydrate chain extension of the same order as that obtained by a 90°C. heat treatment of a composition without wollastonite.

FIG. 9 is a graph obtained from tests of the bonding of treated oruntreated steel wires. Plotted on the y-axis are the values of thepull-out force (kN) and plotted on the x-axis are the values of thedisplacement (mm) of the fibre.

FIG. 10 is a graph obtained from tests of the bonding of steel wires ofdifferent diameters. Plotted on the y-axis are the values of thepull-out force (kN) and plotted on the x-axis are the values of thedisplacement (mm) of the fibre.

FIG. 11 is a graph obtained from tests of the bonding of steel wiresanchored in the concrete over different lengths. Plotted on the y-axisare the values of the debonding stress (MPa) and plotted on the x-axisare the values of the anchoring length (mm).

FIG. 12 is a graph obtained from bonding tests on a concrete accordingto the invention with and without the presence of a defoaming agent.Plotted on the y-axis are the values of the stress (MPa) and plotted onthe x-axis are the values of the deflection (mm) for concrete sampleswith a W/C ratio of 0.24.

FIG. 13 shows the particle size curves of the sum of the constituents(a)+(b)+(c)+(d) for various concretes according to the invention.

FIGS. 14 and 15 give the properties of concretes having differentparticle size distributions.

FIG. 16 is a graph demonstrating the synergy effect between the presenceof bonded fibres and a high-toughness matrix.

It will also be observed that an important characteristic of the presentinvention is to allow concretes to be obtained which have improvedproperties but which contain a markedly smaller amount of metal fibresthan in many proposals of the prior art. In fact, according to theinvention, amounts of metal fibres of less than 4% and preferably ofless than 3.5% of the volume of the concrete after setting, and possiblyespecially of as low as 2% of the volume of the concrete after setting,as illustrated in the above examples, are sufficient to obtain concreteshaving improved mechanical properties. This surprising effect is due, asfar as may be known, to the choice of the constituents of thecomposition of the concrete and of their proportions in thiscomposition.

The examples which follow illustrate the invention without in any waylimiting it.

EXAMPLES

Constituents

In order for the full significance of the comparisons made to be broughtout, the same constituents have been used in the examples and are givenbelow:

Portland cement (a): HTS (high silica content) type from Lafarge(France).

Sand (b): BE31 quartz sand from Sifraco (France).

Quartz flour (b): C400 grade with 50% of the particles smaller than 10microns, from Sifraco (France) or C500 grade with 50% of the particlessmaller than 5 microns, from Sifraco.

Vitreous silica (c): thermal microsilica from the manufacture ofzirconium, of the “MST” type, with a “BET” surface area of 18 m²/g, fromS.E.P.R. (France).

Acicular-type reinforcing particle (d): wollastonite (CaSiO³).

The product used is marketed by Nyco (Nyco Minerals Inc., Willsboro,N.Y., USA) under the name NYAD G, the characteristics of which are:

size: 1 = 300 microns on average (50 microns to 500 microns) d = 20microns;

form factor: l/d=15;

particle size distribution:

<100 US Mesh (%): 99

<200 US Mesh (%): 87

<325 US Mesh (%): 65;

relative density: 2.9.

Reinforcing particle of the “ground” wollastonite type (d):

The product used is the wollastonite NYCO 1250.

The wollastonite NYCO 1250 has an average size (D50) of 8 microns, witha form factor (l/d) of 3 and a particle size of:

<20 microns (%): 100

<10 microns (%): 96.

Flaky reinforcing particle (d): mica (muscovite: hydrated silicate of Aland K).

The product used is marketed by Kaolins d'Arvor, 56270 Ploemeur, France,under the name Micarvor MG 160, the characteristics of which are:

size: 1=75 microns on average (10 microns to 200 microns);

thickness of the flakes: a few microns;

particle size distribution:

<0.160 mm (%): 98

<0.040 mm (%): 30;

relative density: 2.75.

Admixtures:

liquid dispersing agent X 404 from Mapei (Italy) or SSP104 manufacturedby Takemoto Oil (Japan) and distributed by Mitsubishi or OPTIMA 100manufactured and distributed by Chryso;

RHOXIMAT B 36 powder dispersing agent sold by Rhodia Chimie;

RHOXIMAT 6352DD anti-foam agent sold by Rhodia Chimie;

RHOXIMAT CS60SL silica slurry sold by Rhodia Chimie.

Fibres: The metal fibres are steel fibres having a length of 13 mm, adiameter of 200 microns and an ultimate tensile strength of 2800 MPa,supplied by Bekaert (Belgium). When they are present, the fibres areintroduced in an amount of 2% by volume, i.e. a weight relative to thecement of: 0.222.

Preparation of the Concrete Test Pieces

In the examples, the operating method for the manufacture of test piecesconsisted in using a high-turbulence mixer with rotation of thecontainer, of the EIRICH R02 type with a 5 litre capacity, or of theEIRICH R08 type with a 75 litre capacity or a low shear mixer of theHOBART or PERRIER type.

On average, in all the examples, the amount of entrained air is lessthan 3.5%.

Maturing

For the tests, two methods of treating the hardened concrete were used,one with 20° C. maturing and the other with a 90° C. heat treatment.

20° C. maturing: The test pieces are demoulded 48 hours after casting.They are then subjected to a treatment consisting in storing them underwater at 20° C. for a minimum time of 14 days. The test pieces aremachined (if necessary, depending on the test to be carried out) 26 daysafter casting, and the test is carried out 28 days after casting.

90° C. heat treatment: The test pieces are demoulded 48 hours aftercasting. They are then subjected to a heat treatment consisting instoring them in an oven at 90° C. for 24 hours in wet air and then for24 hours in dry air. The optional machining is carried out 6 days aftercasting and the test is carried out at least 7 days after casting.

Measurements

The measurements pertain to the mechanical properties of the matrix,mainly the toughness, and to the mechanical properties of the finalmaterial with the metal fibres, in bending, in tension and incompression.

They are carried out with dimensions of test pieces suitable for thecorresponding measurement.

Toughness

The methods of measuring the toughness of the cementitious matrix are asfollows:

The tests are carried out in 3-point bending, using 40×40×250 or70×70×280 mm notched prisms, i.e. specimens of SENB geometry (the ASTM-E399-83 procedure). A notch with a V-shaped profile is made on theseprisms dry, using a milling cutter equipped with a diamond disc(precision disc with continuous rim). The relative depth a/w of thenotch is 0.4 (a: depth of the notch; w: height of the specimen).

The critical stress intensity factor K_(c) is obtained from the fractureload F and from the length of the crack a at the point of instability(test in displacement control mode, at 10⁻² mm/s, on a SCHENCK universaltesting machine):${K\quad c} = {\frac{3}{2}\quad \frac{F\quad l}{d\quad w^{2}}\sqrt{a\quad Y}}$

where:

1 represents the distance between the support points (bending rig)=200mm,

d and w are the depth and the height of the specimen, respectively,

a is the length of the notch at the instant of fracture,

Y is a shape parameter which depends on the crack length (α=a/w).

In 3-point bending, it is preferred to use the following Y parameter (J.E. Srawley, International Journal of Fracture (1976), Vol. 12, pages 475to 476):$Y = \frac{{1,99} - {\alpha \quad ( {1\quad - \quad \alpha} )\quad ( {{2,15} - {3,93\quad \alpha} + {2,7\quad \alpha^{2}}} )}}{( {1\quad + \quad {2\quad \alpha}} )\quad ( {1\quad - \quad \alpha} )^{3/2}}$

In the case of non-linear behaviour (ductile behaviour), the force Fadopted for estimating the toughness corresponds to the end of thelinear part of the force-displacement diagram; the point of instabilitythen corresponds to the initiation of the crack.

The critical strain energy release rate G, may be obtained from theforce-displacement curves, as long as the contributions due to thespurious strains are removed and the dissipated energy is expressed interms of the ligament cross-section: (w−a)×d.

In plane strain, there is a simple relationship between K_(c) and G_(c):$G_{c} = \frac{K_{c}^{2}\quad ( {1 - \upsilon^{2}} )}{E}$

where:

E is the elastic modulus,

ν represents Poisson's ratio.

E is obtained experimentally by vibrating a prismatic specimen placed ontwo supports, based on the determination of the fundamental frequency(GRINDOSONIC method).

Bonding

With regard to the bonding of the metal fibres in the cementitiousmatrix, the stress is determined by a test involving the extraction of asingle fibre embedded in a concrete block.

The tests were carried out on continuous steel wire having a diameter dof 200 μm.

When the wires are treated, they are carefully degreased(alcohol/acetone) and then pickled (dilute hydrochloric acid). Aphosphatizing-type treatment is then carried out (manganese or zincphosphatizing). Special care is taken at the finishing stage:neutralization, rinsing and drying.

The wires are embedded in 4×4×4 cm concrete blocks. The composition usedis the same as that used for the mechanical test pieces (bending,compression and tension): the water/cement ratio is fixed at 0.25.

The wires embedded over a length of 10 mm are extracted by pulling onthem using a universal testing machine (SCHENCK) at a rate of 0.1mm/min.

The force exerted is measured via a suitable force sensor and thedisplacement of the wire (with respect to the specimen) is measured viaan extensometry sensor.

The average bonding stress is estimated from the following simplifiedformula:$\tau_{d} = \frac{F\quad \max}{\pi \quad \varphi \quad l_{e}}$

where Fmax is the measured maximum force, φ is the diameter of the wireand l_(e) is the embedment length.

Direct Tensile Strength: R_(t)

This is the value obtained in direct tension on dumb-bell test piecesmachined from 70×70×280 mm prisms, i.e. a working section of 70×50 mmover a height of 50 mm. The carefully aligned test pieces are rigidlymounted in the test rig (UTS) with a single degree of freedom.$R_{t} = \frac{F\quad \max}{70 \times 50}$

where Fmax represents the maximum force (peak) in N for a fracturetaking place in the central 70×50 mm section.

The test piece is fixed in the jaws of the tensile testing machine byadhesive bonding and then clamping by means of bolts.

Flexural Strength: R_(f)

R_(f) is the value obtained in 4-point bending on 70×70×280 mm prismatictest pieces mounted on ball supports according to the NFP 18-411 and NFP18-409 standards and ASTM C 1018.${R\quad f} = {\frac{3}{2}\quad \frac{F\quad \max \quad ( {1 - 1^{\prime}} )}{d\quad w^{2}}}$

where Fmax represents the maximum force (peak force) in N, 1=210 mm and1′=1/3 and d=w=70 mm.

Compressive Strength: R_(c)

R_(c) is the value obtained in direct compression on a groundcylindrical specimen (70 mm diameter/140 mm height).${R\quad c} = \frac{4F}{\pi \quad d^{2}}$

where F represents the force in N at fracture and d represents thediameter (70 mm) of the specimens.

Fracture Energy: W_(f)

W_(f) is the value obtained by determining the total area under theforce-deflection curve in a 4-point bending test on 70×70×280 mm prisms.The measured deflection is corrected so as to determine the truedisplacement of the specimen:${W\quad f} = \frac{{\int{F\quad \delta \quad c}}\quad}{d\quad w}$

where F is the applied force, δc is the true displacement (correcteddeflection) and dw is the cross-section of the specimen.

Examples 1 to 17 Influence of the Reinforcing Elements (d)

By way of comparison, the results obtained from concretes in which theconstituents of the composition have been varied and, for some of them,in which certain constituents have been omitted, especially the fibres,have been presented so as to bring out the surprising advantagesresulting from using the combination of the constituents of a concretecomposition according to the invention.

The results of Examples 1 to 17 have been given in Table I below, whichprovides the composition of the concrete specimens produced and theirrespective parameters.

The amounts of the reinforcing elements (d) are given in percentages byvolume with respect to the combined volume of the aggregate particles(b) and of the pozzolanic-reaction particles (c).

The amounts of the other constituents of the concrete (a, b, c,admixture, water) are expressed in parts by weight.

The admixture used in these Examples 1 to 17 is a dispersant.

The sand used is the sand BE31, the particle size distribution of whichis given in Example 24.

TABLE I Example No. 1 2 3 4 5 6 7 8 9 10 Portland cement (a) 1 1 1 1 1 11 1 1 1 Vitreous silica (c) 0.325 0.325 0.325 0.325 0.325 0.325 0.3250.325 0.325 0.325 Quartz flour (b) 0.300 0.300 0.300 0.300 0.300 0.3000.300 0.300 0.300 0.300 Acicular wollastonite (d) 0 0 0.39 0.39 0 0.2400 0 0 0.240 Mica (d) 0 0 0 0 0 0 0.220 0 0 0 Ground wollastonite (d) 0 00 0 0 0 0 0.150 0 0 Sand (b) 1.430 1.430 1.070 1.070 1.430 1.215 1.2151.29 1.430 1.215 Dispersant (solids content) 0.01 0.01 0.02 0.02 0.020.02 0.02 0.02 0.01 0.02 Water 0.200 0.200 0.270 0.270 0.250 0.250 0.3000.250 0.240 0.240 Untreated fibres (volume %) 0 0 0 0 0 0 0 0 2 2Treated fibres (volume %) Maturing or heat treatment (° C.) 20 90 20 9090 90 90 90 90 90 Toughness G (J/m²) 9 10 20 22 13 25 22 15 10 27Flexural strength (MPa) 16.6 16.5 11.1 14.3 21.3 28.7 Tensile strength(MPa) 7.1 6.7 6.0 6.7 10.8 13.0 Compressive strength (MPa) 198.2 201.8182.3 180.3 Example No. 11 12 13 14 15 16 17 Portland cement (a) 1 1 1 11 1 1 Vitreous silica (c) 0.325 0.325 0.325 0.325 0.325 0.325 0.325Quartz flour (b) 0.300 0.300 0.300 0.300 0.300 0.300 0.300 Acicularwollastonite (d) 0 0.240 0 0 0 0 0.240 Mica (d) 0 0 0 0.220 0 0 0 Groundwollastonite (d) 0 0 0 0 0 0 0 Sand (b) 1.430 1.215 1.43 1.215 1.4301.430 1.245 Dispersant (solids content) 0.012 0.015 0.015 0.015 0.0150.015 0.015 Water 0.240 0.240 0.300 0.300 0.250 0.250 0.250 Untreatedfibres (volume %) 2 2 2 2 2 0 0 Treated fibres (volume %) 0 0 0 0 0 2 2Maturing or heat treatment (° C.) 20 20 90 90 90 90 90 Toughness G(J/m²) 10 26 9 24 12 12 29 Flexural strength (MPa) 18.5 25.1 14 25 19 2634 Tensile strength (MPa) 7.7 11.1 N.B.: In Table I, the quantities ofthe constituents of the composition are expressed in parts by weight,the quantity of cement being taken as reference and equal to one part byweight, apart from the fibres, which, when they are present, areindicated as a percentage of the total volume of the composition, D₅₀ =75 μm and D₇₅ = 350 μm

Comparison of Examples 1 and 2 (specimens without wollastonite) withExamples 3 and 4 (specimens with 17% acicular wollastonite) shows analmost twofold increase in the toughness of a concrete not containingmetal fibres. Similar results are obtained by comparing Example 5 (aspecimen without wollastonite) with Example 6 (a specimen with 10%acicular wollastonite), again for a concrete without fibres. Thisimprovement in toughness (addition of wollastonite) depends on thequality and of the nature of the cement.

The toughness of concrete with metal fibres but without wollastonite is10 J/m2 (Example 9) and increases to 27 J/m2 when 10% wollastonite isincorporated (Example 10).

The overall fracture energy results from a cumulative effect of theenergy expended by the matrix (toughness G_(c)) and of the energydissipated by the metal fibres.

It may be seen that the presence of acicular reinforcing particles,especially of wollastonite, in a cementitious matrix of particularly lowporosity, enhances load transfer between the fibres and the concrete,thus making it possible, by virtue of a synergy effect, to take optimumadvantage of the fibres which are present in small amounts with respectto the concrete, and thus to improve the ductility of the material.

This combination of the porosity of the cementitious matrix, of acicularor flaky reinforcing particles and of metal fibres present in smallamounts with respect to the concrete constitutes an important and novelaspect of the present invention.

The anisotropic reinforcing particles thus exert a major role incontrolling microcracking and load transfer between matrix and metalfibres. An improvement in the mechanical properties of the material inbending, tension and compression is also observed.

The use of mica-type flaky reinforcing particles (Example 7) alsoprovides a marked improvement in the toughness.

The use of reinforcing particles of the ground wollastonite type(Example 8) has a positive effect on the toughness of the matrix, but toa lesser extent than in the case of acicular wollastonite.

The introduction of an acicular reinforcing particle results in asignificant increase in toughness; this increases is smaller when theacicularity factor (or size) is reduced.

Similar observations may be made in the case of the other mechanicalproperties. Thus, the use of acicular wollastonite markedly improves theflexural strength: compare Example 11 (no acicular wollastonite) withExample 12 (with acicular wollastonite). The same applies to mica-typereinforcements: compare Example 13 (without mica) with Example 14 (withmica).

In general, the 90° C. heat treatment has a favourable effect on theflexural strength, which is thus improved.

However, even with 20° C. maturing, the flexural strength is increasedby introducing acicular wollastonite (compare Example 12 with Example11, the latter being carried out on a composition without wollastonite).

Moreover, adding acicular wollastonite substantially improves thetensile strength both with 20° C. maturing and with 90° C. heattreatment: in this respect Examples 11 and 15 without acicularwollastonite (control) may be compared with Examples 12 and 17 with 10%acicular wollastonite.

On average, a +25% increase in the intrinsic direct tensile strength offibre-reinforced concrete is observed because of the addition ofwollastonite.

In all the examples, compressive strengths greater than 150 MPa areobtained for concrete compositions having W/C values of less than 0.27.

Moreover, introducing acicular wollastonite improves the uniformity ofthe mechanical properties of the concretes.

This advantageous aspect is illustrated by the graphs in FIG. 1 whichshow, as indicated previously, bending tests carried out on three testpieces of concrete compositions with fibres (W/C=0.24 and 20° C.maturing) that are in all points identical apart from the presence orabsence of reinforcing particles of the acicular wollastonite type. Thecompositions without wollastonite, according to Example 11, give widelyshifted curves (curves 11.1, 11.2 and 11.3), corresponding to a largescatter in the bending results. In contrast, with compositionscontaining wollastonite, namely 10% acicular wollastonite, according toExample 12, the three curves (curves 12.1, 12.2 and 12.3) obtained arevery close together and almost coincident, which means that the scatterin the mechanical properties of the material is almost completelyeliminated.

The same observations apply to the graphs in FIG. 2 relating to testpieces of concretes without wollastonite according to Example 9, (curves9.1, 9.2 and 9.3) and with wollastonite according to Example 10, (curves10.1, 10.2 and 10.3), the concretes tested being concretes with fibreswith a W/C value of 0.24 and a 90° C. heat treatment.

Example 17 relates to a concrete comprising both acicular wollastoniteand treated fibres. It may be seen that the best performance in terms oftoughness and of flexural strength is obtained for this concrete. Thus,it is better than the concrete of Example 10 which comprises onlyacicular wollastonite—and untreated fibres—and better than the concreteof Example 16 which comprises only treated fibres and not acicularwollastonite.

The combination of bonded fibres and a high-toughness matrix really doesresult in improved performance.

It is clearly apparent from the curves in FIG. 5 (Example 1), FIG. 6(Example 2) and FIG. 7 (Example 3) that, in the case of specimens ofconcrete without wollastonite, a low porosity is achieved only if theconcretes are subjected to a heat treatment. On the other hand, theaddition of wollastonite-type reinforcing particles to the compositionof these concretes surprisingly leads to a low porosity, including inthe case of concretes subjected to 20° C. maturing.

The addition of wollastonite thus makes it possible to achieve gooddensification of the concrete (reduced porosity), this being so even inthe case of the normal 20° C. maturing conditions.

Examples 18-23 Influence of the Nature of the Fibres

The above Examples 15 and 16 illustrate already the influence of theimprovement in fibre treatment. Thus, FIG. 4 shows the improvement inthe fibre/matrix bonding obtained by a surface treatment (phosphatizing)of the fibres (curves 16.1, 16.2) compared with untreated fibres (curve15.1), the fibres being incorporated into a matrix as defined in TableI, in Example 15 (untreated fibres) and Example 16 (treated fibres).

Example 18 Treated or Untreated Rods

This example relates to rod-bonding tests carried out using the generalmethod indicated above—except that the steel wires are replaced by steelrods having a diameter d=5 mm.

These rods are introduced into specimens of fibre-free concretes.

The composition of the concrete in parts by weight is as follows:

HTS Portland cement: 1

MST vitreous silica: 0.325

C400 quartz flour: 0.300

BE31 sand: 1.43

dispersant (solids content): 0.02

water: 0.25

The bonding tests were carried out on rods, one being made of untreatedsteel and the other made of steel treated by manganese phosphatizingaccording to the general protocol mentioned above except that these aresteel rods and not steel wires.

With the untreated rod, the average bonding stress measured is 10 MPawhile with the phosphatized rod it is 15 MPa.

Example 19 Treated or Untreated Steel Wires

This example relates to tests of the bonding of steel wires—and not ofrods—carried out using the general method indicated above. The rods areintroduced into specimens of fibre-free concretes having the samecomposition as that of Example 18.

The bonding tests were carried out on wires, one of them made ofuntreated steel and the other of steel treated by zinc phosphatizingaccording to the general protocol mentioned above.

The results are given in FIG. 9. It is clear from this example that thesurface treatment carried out (phosphatizing) leads to a very highbonding level: the shear stress increases from 10 MPa (standard wire) to25 MPa (treated wire)

Example 20 Use of Precipitated Silica to Improve Bonding

This example is intended to illustrate the improvement in thefibre/matrix bonding obtained by modifying the composition of thecementitious matrix of Example 18 by incorporating a precipitatedsilica, the said matrix being used in a concrete with untreated metalfibres, with a W/C value of 0.2 and a 24 h/24 h heat treatment at 90° C.

The results are shown in FIG. 3 which is a graph reproducing the curvesobtained in a tensile test on a 7×7×28 cm test piece for a specimen ofconcrete with 2% by volume of untreated steel fibres, the matrix ofwhich has been modified, or not modified, by adding an amount of thesilica suspension RHOXIMAT CS 960 SL from Rhodia Chimie equal to 1.9% asdry-weight equivalent with respect to the cement (i.e. 0.65% by weightwith respect to the concrete).

FIG. 3 shows the stress to fracture, expressed in MPa, plotted on they-axis and the displacement, expressed in mm, plotted on the x-axis.Curves (20.1, 20.2 and 20.3) give the results for three test pieces withsilica and curves (20.4 and 20.5) for two identical test pieces withoutsilica. It may be seen that the scatter in the results is appreciablyreduced. Furthermore, the energy dissipated after the maximum stress isconsiderably increased.

Example 21 Influence of the Fibre Diameter

This example is intended to illustrate the influence of the fibrediameter on the fibre/matrix bonding.

The composition of the cementitious matrix is that of Examples 18 and19. Steel wires having diameters of 100 and 200 μm were introduced intothis matrix, these being anchored in the matrix over a length of 5 mm.

The results appear in FIG. 10. For an anchoring length of 5 mm, thebonding is clearly higher when the diameter of the wire increases from0.1 mm to 0.2 mm.

Example 22 Influence of the Fibre Anchoring Length

This example is intended to illustrate the influence of the fibreanchoring length on the fibre/matrix bonding.

The composition of the cementitious matrix is that of Examples 18 and19. Steel wires having diameters of 100 and 200 μm were introduced intothis matrix with various anchoring lengths.

The results appear in FIG. 11. For a wire having the givencharacteristics, the bonding level (bonding stress) is constant foranchoring lengths ranging from 5 to 15 mm.

Example 23 Addition of an Anti-foam (or Defoaming) Agent

One means of increasing the bonding of the fibres also consists inadding an anti-foam/defoaming agent to the concrete composition. Thus,Example 16 has been repeated by adding 1% of an anti-foam in solid form(powder).

The results appear in FIG. 12. A gain is observed at the level ofmaximum stress (peak) and especially a greater fracture energy due to abetter quality of fibre/matrix interface.

Examples 25-29 Influence of the Particle Size of the Constituents of theConcrete

Five concretes according to the invention were prepared fromconstituents (a), (b), (c) and (d) having various particle sizedistributions. These particle size distributions are shown in FIG. 13.

It may be seen that, for these 5 concretes, the constituents (a), (b),(c) and (d) satisfy the condition: the D75 particle size is always lessthan 2 mm and the D50 particle size is less than 150 μm. The particlesize distributions differ by the value of the maximum particle size,D100 or Dmax which varies between 500 μm and 6 mm.

Concretes are manufactured from these 5 particle size distributions.Their compositions are given in Table 2. The composition is expressed inpercentage by volume with respect to the entire composition.

TABLE 2 Example 25 26 27 28 29 Dmax (mm) 0.6 1 2.5 4 6 HTS cement (a) 2323 23 22 23 MST silica (c) 10 10 10 10 10 C500 quartz (b) 7 7 7 7 7 BE31sand (b) ⁽*⁾ 37 14 13 8 11 NI 0.4/1.3 sand (b) ⁽*⁾ 0 24 0 0 0 BB 0.5/2.5sand (b) ⁽*⁾ 0 0 25 10 7 BB 2/4 sand (b) ⁽*⁾ 0 0 0 21 0 SK 3/6 sand (b)⁽**⁾ 0 0 0 0 20 NYADG wollastonite (d) 5 5 5 5 5 BEKAERT fibres 2 2 2 22 OPTIMA 100 dispersant 3 3 3 3 3 Water 13 12 12 12 12 ⁽*⁾ SIFRACO ⁽**⁾SILICA and KAOLIN.

The various particle sizes are obtained by varying the nature and theamount of the sands.

The compressive strengths and the flexural strengths in 3-point bendingfor 3 different test pieces of each concrete 25 to 29 are given in FIGS.14 and 15.

It may be seen that, whatever the particle size distribution, andespecially the value of Dmax, the compressive strength remains greaterthan 150 MPa and the flexural strength remains greater than 30 MPa.

Examples 30-33 Effect of the Matrix Toughness/Fibre Bonding Synergy

As indicated in Example 17, there is a synergistic effect between thepresence of bonded fibres associated with a high-toughness matrix.

Examples 30-33 demonstrate this synergy. The basic formula of theseexamples is given in Table 3:

In Example 30, the fibres are steel fibres, wollastonite not beingpresent.

In Example 31, the fibres are steel fibres, wollastonite being present.

In Example 32, the fibres are steel fibres treated by zincphosphatizing, wollastonite not being present.

In Example 33, the fibres are steel fibres treated by zincphosphatizing, wollastonite being present.

The concretes are subjected to a 90° C. cure.

The concretes of Examples 30 to 33 are tested in 3-point bending, theresults appear in curves 30 to 33 of FIG. 16 and the key values aregiven in Table 3 in which the compositions are expressed as weightpercentages relative to the cement.

Example No. 30 31 32 33 Portland cement (a) 1 1 1 1 Vitreous silica (c)0.325 0.325 0.325 0.325 Quartz flour (b) 0.3 0.3 0.3 0.3 Acicularwollastonite (d) 0 0.24 0 0.24 Sand (b) 1.43 1.215 1.43 1.215 Dispersant(solids content) 0.018 0.018 0.018 0.018 Water (w/c) 0.19 0.22 0.19 0.22Untreated fibres (volume %) 2 2 0 0 Treated fibres (volume %) 0 0 2 2Heat treatment (° C.) 90 90 90 90 Yield stress (MPa) 16 28 29 36 Peakstress (MPa) 25 35 37.5 50 Deflection of the peak (mm) 0.8 0.8 1 1.2

The best mechanical properties are obtained in the case of the treatedfibres and the matrix comprising wollastonite of Example 33. it willfurthermore be noted that there is a significant work-hardening effectand a damage mechanism by multi-cracking (network of parallelmicrocracks) and not by mono-cracking.

What is claimed is:
 1. A concrete comprising: a hardened cementitiousmatrix including (a) cement; (b) aggregate particles having a maximumparticle size Dmax of at most 2 mm; (c) pozzolanic-reaction particleshaving an elementary particle size of at most 1 μm; (d) constituentscapable of improving the toughness of the matrix selected from the groupconsisting of acicular and flaky particles, wherein the particles havean average size of at most 1 mm and which are present in a proportion byvolume of between 2.5 and 35% of the combined volume of the aggregateparticles (b) and of the pozzolanic-reaction particles (c); and (e) atleast one dispersing agent; (f) metal fibers dispersed in the hardenedcementitious matrix, wherein (i) the fibers have an individual length 1of at least 2 mm and an 1/d ratio of at least 20, d being the diameterof the fibers, (ii) the ratio R of the average length L of the fibers tothe maximum particle size Dmax of the aggregate particles is at least 10and (iii) the amount of fibers is such that their volume is less than 4%of the volume of the concrete after it has set; and (g) water, whereinthe percentage by weight of water W with respect to the combined weightof the cement (a) and of the particles (c) is in the range 8-24%.
 2. Theconcrete of claim 1, wherein the aggregate particle size is at most 1mm, the pozzolanic-reaction particle size is at most 0.5 μm, and theamount of fibers is such that their volume is less than 3.5% of thevolume of the concrete after it has set.
 3. A concrete comprising: ahardened cementitious matrix including (a) cement; (b) aggregateparticles; (c) pozzolanic-reaction particles having an elementaryparticle size of at most 1 μm; (d) constituents capable of improving thetoughness of the matrix selected from the group consisting of acicularand flaky particles, wherein the particles have an average size of atmost 1 mm and are present in a proportion by volume of between 2.5 and35% of the combined volume of the aggregate particles (b) and of thepozzolanic-reaction particles (c); and (e) at least one dispersingagent, wherein the combination of the constituents (a), (b), (c) and (d)has a D75 particle size of at most 2 mm and a D50 particle size of atmost 200 μm; (f) metal fibers dispersed in the hardened cementitiousmatrix, wherein (i) the fibers have an individual length 1 of at least 2mm and an 1/d ratio of at least 20, d being the diameter of the fibers,(ii) the ratio R of the average length L of the fibers to the D75particle size of the combination of constituents (a), (b), (c) and (d)is at least 5 and (iii) the amount of fibers is such that their volumeis less than 4% of the volume of the concrete after it has set; and (g)water, wherein the percentage by weight of water W with respect to thecombined weight of the cement (a) and of the particles (c) is in therange 8-24%.
 4. The concrete of claim 3, wherein (i) thepozzolanic-reaction particle size is at most 0.5 μm, (ii) the ratio R ofthe average length L of the fibers to the D75 particle size of thecombination of constituents (a), (b), (c) and (d) is at least 10, (iii)the amount of fibers is such that their volume is less than 3.5% of thevolume of the concrete after it has set, (iv) the combination of theconstituents (a), (b), (c) and (d) has a D75 particle size of at most 1mm and (v) a D50 particle size of at most 150 μm.
 5. The concrete ofclaim 1, wherein the toughness of the cementitious matrix is at least 15J/m2.
 6. The concrete of claim 5, wherein the toughness of thecementitious matrix is at least 20 J/m2.
 7. The concrete of claim 1,wherein the particles (d) have an average size of at most 500 μm.
 8. Theconcrete of claim 1, wherein the particles (d) are present in aproportion by volume in the range 5%-25% of the combined volume of theaggregate particles (b) and of the pozzolanic-reaction particles (c). 9.The concrete of claim 1, wherein the particles (d) of acicular shape areselected from the group consisting of wollastonite fibers, bauxitefibers, mullite fibers, potassium titanate fibers, silicon carbidefibers, cellulose or cellulose-derivative fibers, carbon fibers, calciumphosphate fibers, especially hydroxyapatite HAP fibers, calciumcarbonate fibers and derived products obtained by grinding said fibersand mixtures of said fibers.
 10. The concrete of claim 9, wherein theparticles (d) are wollastonite fibers.
 11. The concrete of claim 1,wherein the acicular particles (d) have a length/diameter ratio of atleast
 3. 12. The concrete of claims 11, wherein the acicular particles(d) have a length/diameter ratio of at least
 5. 13. The concrete ofclaim 1, wherein the flaky particles (d) are selected from the groupconsisting of mica flakes, talc flakes, mixed silicate (clay) flakes,vermiculite flakes, alumina flakes and mixed aluminate or silicateflakes and mixtures of the said flakes.
 14. The concrete of claim 13,wherein the particles (d) are mica flakes.
 15. The concrete of claim 1,wherein at least some of the reinforcing particles (d) have, on theirsurface, a polymeric organic coating which comprises a latex or isobtained from at least one compounds selected from the group consistingof polyvinyl alcohol, silanes, siliconates, siloxane resins,polyorganosiloxanes, the product from reaction between (i) at least onecarboxylic acid containing from 3 to 22 carbon atoms, (ii) at least onepolyfunctional aliphatic or aromatic amine or substituted amine,containing from 2 to 25 carbon atoms and (iii) a crosslinking agentwhich is a water-soluble metal complex containing at least one metalselected from the group consisting of zinc, aluminum, titanium, copper,chromium, iron, zirconium and lead.
 16. The concrete of claim 1, whereinthe average bonding stress of the metal fibers in the hardenedcementitious matrix is at least 10 Mpa.
 17. The concrete of claim 16,wherein the average bonding stress of the metal fibers in the hardenedcementitious matrix is at least 15 MPa.
 18. The concrete of claim 1,wherein the fibers are steel fibers.
 19. The concrete of claim 18,wherein the fibers have a variable geometry.
 20. The concrete of claim1, wherein the fibers are fibers which have been etched for the purposeof increasing the bonding of the fiber in the cementitious matrix. 21.The concrete of claim 1, wherein the fibers are fibers on which has beendeposited a mineral compound for the purpose of increasing the bondingof the fiber in the cementitious matrix.
 22. The concrete of claim 1,wherein the fibers have a length between 10-30 mm.
 23. The concrete ofclaim 1, wherein the cementitious matrix further comprises at least onecompounds to increase the bonding of the fibers in the matrix.
 24. Theconcrete of claim 23, wherein the compound is selected from the groupconsisting of silica compounds comprising mostly silica, precipitatedcalcium carbonate, polyvinyl alcohol in aqueous solution, a latex, andmixtures thereof.
 25. The concrete of claim 24, wherein the silicacompound is a precipitated silica introduced with a content of between0.1 and 5% by weight, expressed as dry matter, with respect to the totalweight of the concrete.
 26. The concrete of claim 25, wherein theprecipitated silica is introduced into the composition in the form of anaqueous suspension.
 27. The concrete of claim 26, wherein the aqueoussuspension has: a solids content of between 10 and 40% by weight; aviscosity of less than 4×10⁻² Pa.s for a shear of 50 s⁻¹; and an amountof silica contained in the supernatant liquid of the suspension aftercentrifuging at 7500 rpm for 30 minutes of more than 50% of the weightof the silica contained in the suspension.
 28. The concrete of claim 1,wherein the 1/Ø ratio of the fibers is at most
 200. 29. The concrete ofclaim 1, wherein the maximum particle size Dmax of the aggregateparticles (b) is at most 6 mm.
 30. The concrete of claim 1, wherein theaggregate particles (b) are screened or ground sands or mixtures ofsands.
 31. The concrete of claim 30, wherein the sands comprisesilicious sands or quartz flour.
 32. The concrete of claim 1, whereinthe aggregate particles (b) are present in an amount ranging from 20 to60% by weight of the cementitious matrix.
 33. The concrete of claim 32,wherein the aggregate particles (b) are present in an amount rangingfrom 25 to 50%, by weight of the cementitious matrix.
 34. The concreteof claim 1, wherein the pozzolanic-reaction particles (c) compriseparticles selected from the group consisting of silica compounds, silicafume, fly ash, and blast-furnace slag.
 35. The concrete of claim 1,wherein the percentage by weight of water W with respect to the combinedweight of the cement (a) and of the pozzolanic-reaction particles (c) isbetween 13-20%.
 36. The concrete of claim 1, wherein the concrete ispretensioned.
 37. The concrete of claim 1, wherein the concrete ispost-tensioned.
 38. The concrete of claim 1, wherein the concrete has adirect tensile strength of at least 12 MPa.
 39. The concrete of claim 1,wherein the concrete has a flexural strength in 4-point bending (modulusof rupture) of at least 25 MPa.
 40. The concrete of claim 1, wherein theconcrete has a compressive strength of at least 150 MPa.
 41. Theconcrete of claim 1, wherein the concrete has a fracture energy of atleast 2500 J/m2.
 42. The concrete of claim 1, wherein after it has set,it undergoes maturing at a temperature substantially equal to ambienttemperature.
 43. The concrete of claim 1, wherein, after it has set, itundergoes a heat treatment between 60° C. and 100° C. at normalpressure.
 44. The concrete of claim 43, wherein the duration of the heattreatment is from 6 hours to 4 days.
 45. The concrete of claim 44,wherein the duration of the heat treatment is from 6 h to 72 h.
 46. Apremix for the production of a concrete having fibers disbursed thereincomprising: (a) cement; (b) aggregate particles having a maximumparticle size Dmax of at most 2 mm; (c) pozzolanic-reaction particleshaving an elementary particle size of at most 1 μm; (d) constituentscapable of improving the toughness of the matrix selected from the groupconsisting of acicular and flaky particles, wherein the particles havean average size of at most 1 mm and present in a proportion by volume ofbetween 2.5 and 35% of the combined volume of the aggregate particles(b) and of the pozzolanic-reaction particles (c); and (e) at least onedispersing agent.
 47. The premix of claim 46 further comprising metalfibers having an individual length 1 I of at least 2 mm and an 1/d ratioof at least 20, d being the diameter of the fibers, the ratio R of theaverage length L of the fibers to the maximum particle size Dmax of theaggregate particles being at least
 10. 48. The concrete of claim 21,wherein the mineral compound is silica or a metal phosphate.